Generally, a winch control apparatus for a crane comprises an actuator capable of performing a rotational movement for driving a winch drum, and a brake for braking rotation of the winch drum. The winch control apparatus is configured such that, during stopping of the winch drum, the actuator is stopped, and the brake is activated to restrain movement of the winch drum in rotation directions thereof by a braking force of the brake. The winch control apparatus is also configured such that, when starting a hoisting operation, the braking by the brake is released, and, in response to the release, the actuator performs a rotational movement in a hoisting direction.
Assume a situation where the hoisting operation is started in a state in which a suspended load is stopped in the air. In this case, just after releasing the braking, a torque corresponding to a load (weight load) from the suspended load will be suddenly applied to the actuator. In particular, when the load from the suspended load is large, the actuator can fail to resist the suddenly-applied torque, thereby leading to occurrence of a reverse rotation phenomenon that the actuator is temporarily rotated reversely in an unwinding direction of a wire rope (lowering direction).
As a means to avoid such temporary falling of the suspended load due to the reverse rotation phenomenon of the actuator, there is a technique disclosed in JP 2001-165111A. Specifically, the JP 2001-165111A discloses a control apparatus for a hydraulically-driven winch comprising a reverse rotation prevention means operable, when switching a rotation direction switching valve to a hoisting position, to immobilize an lowering-directional rotation of a drive motor for driving a winch dram, until a drive pressure of the drive motor is boosted to cause the drive motor to start rotating in a hoisting direction. In this control apparatus, the reverse rotation prevention means is composed of a device which comprises a ratchet wheel for immobilizing a rotary shaft of the drive motor, a pawl insertable between adjacent teeth of the ratchet wheel, a cylinder for selectively moving the pawl forwardly and backwardly, and a pilot switching valve for introducing a control pressure into the cylinder.
Further, as a means for a motor-driven winch apparatus to prevent a temporary falling of a suspended load, there is a technique disclosed in JP 2002-46985A. Specifically, the JP 2002-46985A discloses a crane comprising: a suspended load holding torque calculation section for estimating the weight of a suspended load from a torque current and the speed of the suspended load, and calculating a suspended load holding torque based on the estimated weight of the suspended load; a maximum torque calculation section for calculating a maximum torque outputtable by a motor; and a control section for calculating an acceleration torque of the suspended load by subtracting the suspended load holding torque from the maximum torque, and subjecting the motor to acceleration control, based on the calculated acceleration torque.
However, the technique disclosed in the JP 2001-165111A requires adding the aforementioned reverse rotation prevention means to a crane, so that there is a problem of an increase in the number of component, leasing to increase in cost, deterioration in reliability and increase in size of the apparatus.
In the technique disclosed in the JP 2002-46985A, there is a possibility that, due to an estimate error in the estimated weight of the suspended load, the suspended load holding torque is estimated to be smaller than an actual suspended load holding torque. In this regard, the technique disclosed in the JP 2002-46985A is not configured to calculate an additional torque for compensating for such an insufficient torque. Therefore, falling of the suspended load can occur.